This invention relates to optical wave power control devices and methods, and more particularly to systems, devices and methods for modulating and switching signals transmitted in optical waveguides.
In the now rapidly expanding technology of fiber optics, a number of discrete devices and subsystems have been developed to modulate, or otherwise control, optical beams that are at specific wavelengths. The approaches heretofore used, however, have not fully overcome one or more problems inherent in the requirements imposed by modern systems. Present day communication systems increasingly use individual waveguide fibers to carry densely wavelength multiplexed optical beams, and modulate the beams at very high digital data rates or with wideband analog data, or both.
For example, it is known how to modulate the power of a monofrequency laser source, typically a semiconductor laser. Using such a source, one must accept a limited modulation bandwidth because of constraints on the rate at which the laser can be turned on and off. In addition, this type of modulation introduces chirping, or spreading of the bandwidth of the signal from the monofrequency laser, so that dispersion variations with wavelength in signals that are transmitted in optical fiber over a substantial distance place an inherent limit on that distance. This approach does have the advantage, as compared to some other systems, of modulating at the source, so that continuity in the optical fiber structure can be preserved. However, semiconductor lasers that are modulated must be coupled to optical waveguides by means which introduce problems with yield, reliability and cost. Consequently, the limitations mentioned above are such that long distance transmission systems tend to employ external modulators.
The two forms of external modulators that are currently employed are monolithic waveguide devices. A widely used lithium niobate modulator of this type is based on a Mach-Zehnder interferometer and is being employed in long distance transmission systems and other applications because it creates clean waveforms at the highest data rates and produces a minimal amount of chirping. As a monolithic waveguide device, it must be coupled at its input and output to an optical fiber, which requires costly packaging and assembly but even so introduces a substantial mismatch between the chip waveguide and the optical fiber waveguide, thus entailing losses in the range of about 5 db. Furthermore, it is polarization sensitive and must be actively temperature stabilized to compensate for the thermal drift characteristics of the interferometer
A second waveguide device, more recently introduced, is also a monolithic on-chip device using an electro-absorption effect. This modulator is fabricated integrally with a semiconductor laser, requiring sophisticated and costly fabrication technology that inevitably decreases the yield of the overall laser device. In addition, such a device is subject to chirping, which places a limitation on high (10 gigabit/sec and higher) modulation rates. The integral laser/modulator chip must be coupled to optical fiberxe2x80x94again adding cost to manufacturing.
There are a number of other patents of recent interest which disclose variants on the monolithic device structure, but all require a matching technique to be used to function with an optical fiber. Mention of signal modulation is made in at least two patents which often employ dielectric microcavities for recirculating electromagnetic wave energy at optical wavelengths. xe2x80x9cWhispering gallery modexe2x80x9d (WGM) structures, which comprise microresonators of generally spherical, ring, or disc-like configuration, are of dielectric material, e.g. glass or silica. They are essentially totally reflective and support internal modes at frequencies determined by size and other factors, with very low losses, and therefore high Q. They are being investigated for use in a number of different optical configurations. U.S. Pat. No. 5,343,490 to McCall, for example, discloses a closed loop WGM system configured as a thin element, described as xe2x80x9can active material element of thickness characteristically of a maximum of a half wavelength . . . xe2x80x9d (Col; 1, lines 62-63). Disks are described that have thicknesses in the range of 1,000-1,500 xc3x85 and have at least one optically active layer, sandwiched between thicker barrier layers. The optically active material may be InGaAs and the barrier layers InGaAsP material, for example. Fabricated into a microcavity using photolithographic techniques, the structure is described as having multiple potential functions. These comprise optically pumped single quantum well to multiple quantum well structures and various two port and three port devices which may function as, for example, detectors, data amplifiers, and current meters. It is mentioned in passing, as at Col. 6, lines 3-23, that the output may be modulated or unmodulated, but apart from general statements (e.g., xe2x80x9cdelicate destructive phase interferencexe2x80x9d in terms of canceling an unmodulated output) there is no teaching as to how modulation, much less high speed modulation could be effected. Continued evolution of this approach may lead to practical modulators at some point in time, but even then would face the barriers presented by the need for matching to fiber waveguide structures, and in cost, and in performance specifications such as insertion loss.
A somewhat related approach is described in the xe2x80x9cPhotonic Wire Microcavity Light Emitting Devicesxe2x80x9d application of Ho, et al. in U.S. Pat. No. 5,878,070. The inventors also describe a WGM microcavity with a gain medium of InGaAs sandwiched between InGaAsP layers of submicron thickness, but closely surround a ring of this optically active structure with an arc of lower refractive index waveguide material in a general U-shape, the side arms of which may be tapered (FIG. 9). With this arrangement, there is resonant photon tunneling from the active material of the gain cavity to the output-coupled waveguide, which serves as the core of the structure. The possibility of modulation, by varying the pumping power of the active medium section, is also suggested, (Col. 15, lines 54-58) with no specific implementation being described. Since the concept is based upon a discrete and particular active waveguide core and an arc of low refractive wave index material serving as an output waveguide in close association to it, is evident that the same problems that are presented by the McCall disclosure are also present here.
In addition to the rapidly increasing use of fiber optic systems, there is constant evolution toward denser wavelength division multiplexing and higher data rates per channel. This in turn means that factors such as spectral bandwidth, frequency stability, compactness and reproducibility are of added importance, and place added requirements on any new approach.
An all fiber modulator, one that assures the continuity of the wave energy transmitted along an optical waveguide, will therefore be of substantial potential benefit, if it can be provided in a form that offers sufficient dynamic range, and minimizes insertion losses while being capable of handling high data rates. It is evident that such a device, if wavelength sensitive, can also be used as an on-off switch, or a switchable bandpass filter, where required for specific applications. Preferably, for complex switching and routing systems having many channels, units using the same concepts can be fabricated using microlithographic or micromachining techniques.
These and other objectives of the invention are met by a power transfer structure and modes of operation which variably attenuate (modulates) or completely block (switches off) the power propagated in a section of an optical waveguide. To this end a short section of an optical waveguide is modified to couple power into an adjacent high Q resonator microcavity in which wave energy of a resonant mode recirculates with power accumulation before return to the waveguide. In a first possible mode of operation, the optical losses upon one round trip in the resonator are such that resonator to wave-guide coupling losses are greater than other resonator losses. This is referred to as an over-coupled condition, under which condition the resonator minimally attenuates resonant optical power incident from the wave guide resulting in maximal waveguide transmission. By increase of the resonator loss per round trip (with resonator to wave guide coupling loss fixed) to bring it into balance with resonator to wave guide coupling loss, the condition goes from one of over coupling to critical coupling, a condition in which wave guide power transmission is zero. The transmission along the waveguide is thereby modulated from essentially unity to essentially zero. This requires a very small change in the round-trip loss induced by a control element, which may be external to the resonator or alternatively based upon varying a property of the resonator itself. Such modulation provides very high data rate capability with an all waveguide transmission structure that involves no discontinuities and requires no coupling of dissimilar elements and has minimal insertion loss. Operation between a critical coupling condition and an undercoupled condition is also feasible for the purpose of modulation. In this second mode of operation round-trip resonator to wave-guide coupling loss is in balance with resonator losses before increase of the resonator loss by the control element. In this condition wave guide transmission is zero as described above. By increase of the resonator loss beyond the condition of balance a condition of under-coupling is obtained in which wave-guide transmission is restored to a value approaching unity transmission. Both the first and second modes of operation can also be realized using negative optical loss (or optical gain), however, the sense in which the optical gain is applied is opposite to that for positive optical loss. For example, in the first mode of operation, the losses would be such that a condition of critical coupling exists prior to application of the optical gain. The control element would then apply optical gain to achieve a condition of over-coupling, thereby modulating the transmission from essentially zero to essentially unity.
Third and fourth modes of operation parallel the first and second modes of operation in that variation between conditions of over coupling and critical coupling (mode 1 and mode 3) or between conditions critical coupling and under coupling (mode 2 and mode 4) is used to modulate wave-guide transmission. However, in these modes of operation, the resonator to wave guide coupling loss is varied (as opposed to being held fixed) while the other resonator losses are held. fixed. The control element in these cases effects a variation in the resonator to wave guide coupling loss. Otherwise, the principle of operation is essentially the same as that for modes 1 and 2.
In a fifth mode of operation, the losses are such that the resonator is critically coupled to the wave guide. The optical path length of the resonator is then varied to shift the resonant frequency of the resonator into or away from resonance with the desired optical wave and thereby effect modulation. Optical path length variation can be achieved, for example, by electrooptic or nonlinear optical induced variation of the resonator dielectric constant.
Since the combined elements are very small and frequency specific a number of units can be used in combination with separate controls for dense wavelength division multiplexing. Switching systems and multiple modulation arrangements, with or without in-fiber signal sources or amplifiers, can be arrayed as needed for particular applications.
Further in accordance with the invention the optical waveguide or fiber may comprise a known core-cladding structure tapered down to a short section of much smaller cross-section. In this section the fiber has only a vestigial core, and power is confined within the reduced cladding and a limited radius of the surrounding environment. The WGM resonator periphery is within the external field in the narrow waist region providing a field coupling and the resonance geometry provides an equatorial internal surface that has essentially total internal reflection and/or wave guiding effect. This establishes a high Q wave recirculation path within an internal circumference of the resonator. The field coupling transfers power into the resonator, which itself does not fully confine the waves, and a part of the power returns to the waveguide as output. A loss control mechanism on, within, or adjacent to the resonator and influencing the exterior or interior fields introduces further loss, the value of which affects the power transmitted through the fiber. The loss control mechanism may advantageously be any form of transducer having a signal variable optical transmissivity characteristic at the chosen wavelength. As one example, an optically active combination of layers of semiconductor materials positioned on or near the resonator is of convenient size, efficiency and signal responsiveness for the desired control. These materials could be bulk or quantum well materials and their absorption varied by a photo pumping, injection current, or applied voltage. As another example, a variable coupling mechanism that couples resonator power to a separate structure such as another wave guide could be positioned to couple power from the resonator and thereby vary its roundtrip loss.
The resonator element is conveniently a silica microsphere, disc, or ring sized to have resonant modes at one or more chosen wavelengths, and of the order of about 1 to 1000 microns in diameter. Advantageously the equatorial diameter is selected with respect to data rate and spectral linewidth, as well as Q, and very small diameters (e.g. 30 microns) are needed for present and anticipated requirements. Likewise resonator shape and size affect the frequency separation between adjacent resonator modes. This frequency separation must at a minimum exceed the desired modulation rate or signal bandwidth, however, in practice it must be wide enough to encompass the spectral extent of optical waves co-propagating in the wave guide. To this end, eccentric resonator structures are desirable such as oblate spheroids, discs, rings and oblongs. To be positioned and held in proper relation to the fiber waist, which may be of less than 10 micron diameter, it can be attached directly, with, for example, the controllable loss transducer being on the opposite side from the fiber.
Both theory and practice establish that the effective range of loss control that is to be observed need vary only between an overcoupled condition in which transmission is unity, or only slightly less, and a critical coupling condition in which transmission is attenuated by in excess of 90%. Because this results, in real terms, from only a small change in applied loss by a loss control mechanism, this approach is therefore preferred to operation between a critical condition and an undercoupled condition and to operation in which criticality is fixed while resonant frequency is varied. In the latter cases different dynamic ranges must be recognized as to both control and power.
The modulator is polarization sensitive, which is typically not of importance when it can be placed close to a source laser which provides a polarized output. Where it is desired to provide polarization insensitivity, two resonators, such as silica microspheres, can be disposed in orthogonal positions relative to the central axis of the fiber. The geometry of the resonator itself, as well as the material used, can be varied as long as the desired Q value and resonator modal frequency separation is maintained. Thus oblate, ring, disc, elliptical, oblong, annular and polygon shapes, among others, are known and can be employed in this application.
To utilize the concepts for concurrent modulation of different wavelength signals multiplexed on the same fiber, it is merely required to dispose a series of resonator/loss controller combinations along one narrow waist section, or along separate taper sections of the fiber. Each resonator is responsive only to its own chosen wavelength and the wavelengths are separately modulated with minimal cross-talk. In-fiber laser sources, such as DFB fiber lasers, can also be employed in the series, adding optical pumping in co-directional or counter-directional relation. The integration of multiple resonator-based modulators in a wavelength division multiplex system provides a wavelength addressable transmission system.
As described above, for concurrent modulation and for wavelength specific modulation of one co-propagated wave with other waves, an appropriate frequency separation between adjacent resonances is established to prevent unintended interference effects. Further the adjacent modal frequency separations within resonators, which support multiple modes at different frequencies, are arranged to exceed the total bandwidth of a frequency range of interest, such as that spanned by the number of WDM channels on the waveguide fiber. Resonator geometries are adaptable to meet these requirements.